Carrier doping to a partially disordered state in the periodic Anderson model on a triangular lattice

We investigate the effect of hole and electron doping to half-filling in the periodic Anderson model on a triangular lattice by the Hartree-Fock approximation at zero temperature. At half-filling, the system exhibits a partially disordered insulating state, in which a collinear antiferromagnetic order on an unfrustrated honeycomb subnetwork coexists with nonmagnetic state at the remaining sites. We find that the carrier doping destabilizes the partially disordered state, resulting in a phase separation to a doped metallic state with different magnetic order. The partially disordered state is restricted to the half-filled insulating case, while its metallic counterpart is obtained as a metastable state in a narrow electron doped region.Comment: 4 pages, 2 figure

Thermal Properties of Heavy Fermion Compound YbP

Low-temperature specific heat and its field-dependence up to 16 T was measured in a stoichiometric single crystal of YbP. A sharp peak was observed at {\it T}$_{\rm N}$ = 0.53 K in zero magnetic field. Application of external field seems to induce a new magnetic phase above 11 T. The field dependence of the transition temperature in the high-field phase is different from that of the low field phase. The linear coefficient of the electronic specific heat is estimated as 120 mJ/mole K$^{2}$ from low temperature specfic heat, suggesting heavy Fermion state in YbP.Comment: to be published in J.Phys.Soc.Jpn on May, 200

Spiral magnetic structure in spin-5/2 frustrated trimerized chains in SrMn3P4O14

We study a spin-5/2 antiferromagnetic trimerized chain substance SrMn3P4O14 using neutron powder diffraction experiments. The coplanar spiral magnetic structure appears below T_N1 = 2.2(1) K. Values of several magnetic structure parameters change rapidly at T_N2 = 1.75(5) K, indicating another phase transition, although the magnetic structures above and below T_N2 are the qualitatively same. The spiral magnetic structure can be explained by frustration between nearest-neighbor and next-nearest-neighbor exchange interactions in the trimerized chains.Comment: submitted to Phys. Rev.

Partial Disorder in the Periodic Anderson Model on a Triangular Lattice

We report our theoretical results on the emergence of a partially-disordered state at zero temperature and its detailed nature in the periodic Anderson model on a triangular lattice at half filling. The partially-disordered state is characterized by coexistence of a collinear antiferromagnetic order on an unfrustrated honeycomb subnetwork and nonmagnetic state at the remaining sites. This state appears with opening a charge gap between a noncollinear antiferromagnetic metal and Kondo insulator while changing the hybridization and Coulomb repulsion. We also find a characteristic crossover in the low-energy excitation spectrum as a result of coexistence of magnetic order and nonmagnetic sites. The result demonstrates that the partially-disordered state is observed distinctly even in the absence of spin anisotropy, in marked contrast to the partial Kondo screening state found in the previous study for the Kondo lattice model.Comment: 4 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp

Partial Disorder and Metal-Insulator Transition in the Periodic Anderson Model on a Triangular Lattice

Ground state of the periodic Anderson model on a triangular lattice is systematically investigated by the mean-field approximation. We found that the model exhibits two different types of partially disordered states: one is at half filling and the other is at other commensurate fillings. In the latter case, the kinetic energy is lowered by forming an extensive network involving both magnetic and nonmagnetic sites, in sharp contrast to the former case in which the nonmagnetic sites are rather isolated. This spatially extended nature of nonmagnetic sites yields a metallic partially-disordered state by hole doping. We discuss the mechanism of the metal-insulator transition by the change of electronic structure.Comment: 4 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp

New structural and magnetic aspects of the nanotube system Na2V3O7

We present new experimental results of low temperature x-ray synchrotron diffraction, neutron scattering and very low temperature (mK-range) bulk measurements on the nanotube system {\tube}. The crystal structure determined from our data is similar to the previously proposed model (P. Millet {\it et al.} J. Solid State Chem. $\bf{147}$, 676 (1999)), but also deviates from it in significant details. The structure comprises nanotubes along the c-axis formed by stacking units of two V-rings buckled in the $ab$-plane. The space group is P$\bar{3}$ and the composition is nonstoichiometric, Na(2-x)V3O7, x=0.17. The thermal evolution of the lattice parameters reveals anisotropic lattice compression on cooling. Neutron scattering experiments monitor a very weak magnetic signal at energies from -20 to 9 meV. New magnetic susceptibility, specific heat measurements and decay of remanent magnetization in the 30 mK - 300 mK range reveal that the previously observed transition at ~76 mK is spin-glass like with no long-range order. Presented experimental observations do not support models of isolated clusters, but are compatible with a model of odd-legged S=1/2 spin tubes possibly segmented into fragments with different lengths

Neutron scattering study of Kondo lattice antiferromagnet YbNiSi3

The Kondo lattice antiferromagnet YbNiSi3 was investigated by neutron scattering. The magnetic structure of YbNiSi3 was determined by neutron diffraction on a single-crystalline sample. Inelastic scattering experiments were also performed on a pulverized sample to study the crystalline electric field (CEF) excitations. Two broad CEF excitations were observed, from which the CEF parameters were determined. The temperature dependence of the magnetic susceptibility chi and the magnetic specific heat Cmag were calculated using the determined CEF model, and compared with previous results.Comment: 10 pages, 7 figures,submitted to JPS

Negative magnetization of Li2Ni2Mo3O12 having a spin system composed of distorted honeycomb lattices and linear chains

We study themagnetism of a spin-1 substance Li2Ni2Mo3O12. The spin system consists of distorted honeycomb lattices and linear chains of Ni2+ spins. Li+ ions enter about 25% and 50% of the honeycomb and chain Ni sites, respectively, creating disorder in both spin subsystems. A magnetic phase transition occurs at Tc = 8.0 K in the zero magnetic field. In low magnetic fields, the magnetization increases rapidly below Tc, decreases below 7 K, and finally becomes negative at low temperatures. We determine the magnetic structure using neutron powder diffraction results. The honeycomb lattices and linear chains show antiferromagnetic and ferromagnetic long-range order, respectively. We investigate static and dynamic magnetic properties using the local probe technique of muon spin relaxation. We discuss the origin of the negative magnetization

Ferroquadrupole ordering and Gamma_5 rattling motion in clathrate compound Ce_3Pd_20Ge_6

Lattice effects in a cerium based clathrate compound Ce_3Pd_20Ge_6 with a cubic Cr_23C_6-type structure have been investigated by ultrasonic and thermal expansion measurements. Elastic softenings of (C_11-C_12)/2 and C_44 proportional to the reciprocal temperature 1/T above T_Q1 = 1.25 K are well described in terms of the quadrupole susceptibility for the ground state Gamma_8 quartet. A huge softening of 50 % in (C_11-C_12)/2 and a spontaneous expansion DL/L = 1.9x10^-4 along the [001] direction in particular indicate the ferroquadrupole ordering of O_2^0 below T_Q1. The elastic anomalies associated with the antiferromagnetic ordering at T_N2 = 0.75 K and the incommensurate antiferromagnetic ordering are also found. Notable frequency dependence of C_44 around 10 K is accounted for by the Debye-type dispersion indicating a Gamma_5 rattling motion of an off-center Ce ion along the [111] direction with eight fractionally occupied positions around the 4a site in a cage. The thermally activated Gamma_5 rattling motion obeying a relaxation time t = t_0exp(E/k_BT) with an attempt time t_0 = 3.1x10^-11 sec and an activation energy E = 70 K dies out with decreasing temperature, and then the off-center tunneling state of Ce ion in the 4a-site cage will appear at low temperatures.Comment: 11 pages, 15 figures, to be published on Phys. Rev.